The present invention relates to a proximity detector employing a capacitive sensor.
In a numerous industrial applications, the proximity between a machine and an obstacle must be detected and measured, whether this is another object or an individual, in order to deliver proximity distance information, alarm signals and to act as a result.
By way of non-limitative example, the industrial application of this type of detector which can be mentioned is the management of an anti-collision function between mobile or stationary robots and an obstacle, management of an anti-burglary function and, more generally, any management implementing proximity detection.
In the medical field, various robots used for the auscultation of patients need to know the position of the patient relative to the moving parts of the machine.
By way of example, for applications in radiology or in imaging, or also for a medical or surgical treatment, it is essential to be able to provide the operator of a piece of equipment or automated control systems with information which is as exact as possible concerning the position of the patient in order to position the auscultation elements rapidly and correctly.
As regards radiology systems employing X-rays, knowledge in real time to a matter of millimeters of the position of a piece of radiology equipment relative to a patient and its immediate hardware environment would allow the speed of the machine's movements to be increased, enhance safety, and minimise exposure times to X-rays.
To increase the speed of movement of vascular positioners, at the same time as guaranteeing no collision with the patient is one of the current aspirations. However, as the physiognomy of the patient and his position relative to the reference frame of the machine is unknown, the speeds at which these robots move are low in order that the moving parts of the machine do not accidentally injure the patient. Generally, an emergency shutdown comprising mechanical circuit breakers stops all movement when the X-ray detector or emitter comes into contact with the patient or another part of the equipment; however, the kinetics of moving objects and the short travel of the contactors requires low displacement speeds. Increasing the speed of the robot is only possible if a non-contact device detects the patient at a distance, termed the upper threshold, adequate to slow down the movements before coming into contact with the patient. A minimum distance, termed the lower threshold, allows the anti-collision emergency shutdown function to be carried out.
Currently, there is therefore a genuine requirement for non-contact proximity detectors providing accurate distance information which can be used in specific environments such as that of medical imaging. The documents U.S. Pat. No. 4,987,583, WO 9730633 and WO 9719638 disclose proximity detectors suited to this type of application.
The document U.S. Pat. No. 5,982,835 discloses a non-contact proximity detector. The electronics used comprise an all-or-nothing detector functioning with a FET transistor oscillator connected to an unshielded measurement electrode.
The document U.S. Pat. No. 5,442,347 discloses a proximity detector of the capacitive type with controlled double shielding, functioning in phase difference measurement mode, while using RC constants generated with reference resistances. A shield is created by reproducing the sensor signal using a buffer. However, a major theoretical problem becomes apparent in this concept, as the buffer adds a parasitic capacitance to the capacitance to be measured. This parasitic capacitance is much greater than the capacitance to be measured, which leads to measurement errors and significant instabilities.
The document U.S. Pat. No. 5,554,973 discloses an electrostatic detector of the capacitive type, operating in accordance with a switched capacitance operating principle, without a shield.
The document U.S. Pat. No. 6,348,862 discloses a proximity detector which includes a detection electrode and a plurality of control electrodes situated close to a spatial region in which an object to be detected is sited.